1. Field of the Invention
The present invention relates to the field of power electronics. It concerns a voltage converter having a voltage intermediate circuit and an invertor following on from the voltage intermediate circuit, which invertor produces a three-phase AC voltage at its outputs and is connected to the output terminals of the voltage converter via a filter arrangement, the filter arrangement having in each case a filter inductor, connected in series with the output terminal, and a filter capacitor which is connected between the output terminal and a common capacitor star point.
Such a voltage converter is disclosed, for example, in the printed publication EP-A1-0 682 401 (FIG. 1).
2. Discussion of Background
In order to achieve generally loss-free filtering of the output voltage from a converter having a voltage intermediate circuit (voltage converter), use is nowadays made of a filter comprising filter inductors and filter capacitors (LC filter). A possible embodiment is one which has filter capacitors connected in star formation, as described in the printed publication mentioned in the introduction and illustrated in FIG. 1. In this case, the voltage converter 39 shown in FIG. 1 comprises a voltage intermediate circuit 11 having two terminals 12 and 13, between which intermediate circuit capacitors 14, 16 are arranged and have a center tap 15 or else can be one capacitor. The voltage intermediate circuit 11 is followed on by an invertor 10 (equipped with power semiconductors) which outputs the generated polyphase AC voltage at corresponding output terminals 20, 21 and 22 via a filter formed from filter inductors 17, 18 and 19 and filter capacitors 23, 24 and 25. The filter inductors 17-19 are in each case connected in series with the output terminals 20-22. The filter capacitors 23-25 are connected between the output terminals 20-22 and a common capacitor star point 26.
In another known filter circuit, which is shown in FIG. 2, the filter capacitors 27, 28, 29 are delta-connected, i.e. connected between two of the output terminals 20-22 in each case.
In a further filter circuit, which is shown in FIG. 3 and is likewise disclosed in the printed publication mentioned in the introduction (FIG. 3), three respective filter capacitors 30-32 and 33-35 jointly provide feedback from the output terminals 20-22 to the terminals 12 and 13 of the voltage intermediate circuit 11.
The circuits shown in FIGS. 1 and 2 filter only the pure three-phase voltage systems. The DC system, on the other hand, is not filtered. Sudden voltage changes in the DC system are passed on unimpeded to the connection terminals 20-22 of the load if the star point of the load or, in FIG. 1, the capacitor star point 26 is not grounded. The two measures cannot be used in a general manner, however.
The circuit shown in FIG. 3 does not have these disadvantages. Its own disadvantage is of a system-related nature and comes to bear when the conditions of use for the filter are such that, with no additional measures, free oscillations are continually excited. Loss-free attenuation of these free oscillations can be achieved only by using a controller which intervenes in the switching sequence of the invertor 10. Nowadays, the switching sequence of the invertor 10 is usually determined by a control computer which uses the load conditions as a basis for establishing the switching commands for the power switches in the invertor. The control computer usually operates in only two coordinates, on account of the symmetry of the output. This makes it possible to determine the switching states of the three phases. From the viewpoint of the filter in the circuit shown in FIG. 3, the three phases are decoupled. Any resonance control must therefore be carried out with three control loops which act independently. This means that it is no longer a simple matter to integrate resonance correction into load control. However, as both parts of the control have access to the same manipulated variable, they must not operate in a decoupled manner. In all cases, a considerable amount of additional effort is required in order to resolve this conflict. A further disadvantage of the circuit shown in FIG. 3 is the high additional current loading of the intermediate circuit capacitors and the invertor, which is caused by the filter capacitors being subjected to charge reversal by the DC voltage system, particularly at low switching frequencies of the invertor semiconductor switches. The same disadvantages also arise if the capacitor star point 26 in the circuit shown in FIG. 1 is connected directly to one of the terminals 12, 13 or the center tap 15.